1. Field of the Invention
The present invention relates to methods for characterizing interface devices, such as adapters, test fixtures and other networks, used with a vector network analyzer (VNA). More particularly, the present invention relates to methods for determining scattering-parameters (S-parameters) of such interface devices.
2. Description of the Related Art
A vector network analyzer (VNA) is an instrument that is typically used to measure complex transmission and reflection characteristics of devices under test (DUTs). Different DUTs can have different types of connection configurations at their ports. For example, a two-port DUT can have one of many of possible configurations of connectors at its two ports. Some DUTs, known as xe2x80x9cinsertablexe2x80x9d DUTs, have two connectors that are from the same connector family and of opposite sex, one connector being male and the other being female. An insertable two-port DUT is configured such that calibration can be performed by connecting ports of a VNA together with the aid of a cable to establish a thru-connection, during calibration, and without having to change the configuration measurement setup for the actual measurement of the DUT.
In contrast, a xe2x80x9cnon-insertablexe2x80x9d DUT cannot be connected to the two ports of a VNA without use of adaptors or other test fixture arms (referred to collectively hereafter as interface devices). An example of a non-insertable device is a xe2x80x9creversible device,xe2x80x9d which has two connectors of the same family, but also of the same sex (i.e., both being male or female). Another example of a non-insertable DUT is a transitional device that has two connecters that are of different families (e.g., one connector being a coaxial cable and the other being a waveguide). Stated generally, interface devices (e.g., an adapters or test fixture arms) are required when using a VNA to make measurements of certain devices (e.g., non-insertable devices).
When interface devices are used to connect DUTs to a VNA, there is a need to remove the effects of such interface devices. This can be accomplished by determining the scattering-parameters (S-parameters) of the interface devices in question and then de-embedding (i.e., mathematically removing) them from measured data. The present invention relates to characterizing of interface devices. Accordingly, the focus hereafter shall be directed to characterizing interface devices, and any discussion of de-embedding (methods of which are well known in the relevant art) will be limited.
S-parameters of a multi-port device characterize how the device interacts with signals presented to the various ports of the device. An exemplary S-parameter is xe2x80x9cS12.xe2x80x9d The first subscript number is the port that the signal is leaving, while the second is the port that the signal is being injected into. S12, therefore, is the signal leaving port 1 relative to the signal being injected into port 2. The four S-parameters associated with an exemplary two-port device 102 are represented in FIG. 1, where:
S11 is referred to as the xe2x80x9cforward reflectionxe2x80x9d coefficient, which is the signal leaving port 1 relative to the signal being injected into port 1;
S21 is referred to as the xe2x80x9cforward transmissionxe2x80x9d coefficient, which is the signal leaving port 2 relative to the signal being injected into port 1;
S22 is referred to as the xe2x80x9creverse reflectionxe2x80x9d coefficient, which is the signal leaving port 2 relative to the signal being injected into port 2; and
S12 is referred to as the xe2x80x9creverse transmissionxe2x80x9d coefficient, which is the signal leaving port 1 relative to the signal being injected into port 2.
A number of procedures have been developed for determining the S-parameters of interface devices (e.g., adapters). In a first technique, referred to hereafter as method A, S-parameters are determined using a simple one port calibration and some measurements with an interface device in place. Method A (often known as the xe2x80x9creflective-terminationxe2x80x9d method) shall be described with reference to FIG. 2, which shows a first port of a two-port interface device under test 204 coupled to a calibration port of a VNA 202 and a second port of interface device under test 204 coupled to reflective standards 206. Reflective standards 206 normally have reflective coefficient magnitudes of unity and phases differing by pi (xcfx80). Method A, however, can only determine one reflection coefficient (i.e., S11). Further, the high dependence on the standards quality (i.e., the quality of reflective standards 206), among other reasons, tends to increase the uncertainty of this method and can lead to larger errors than may be acceptable.
A second technique, referred to hereafter as method B, is similar to method A but uses multiple line lengths after the interface device to acquire more information. Method B (often known as the xe2x80x9cmultiline one-portxe2x80x9d method), illustrated in FIGS. 3A and 3B, can be used to extract all S-parameters and is less dependent on standards quality. However, method B is relatively complicated, even for a single port. In method B, a VNA 302 is used to make a series of measurements of several standards (typically a load 306 plus several line lengths 308, 310 with reflective standards 312, 314 attached) both with an interface device under test 304 present (as shown in FIG. 3B) and absent (as shown in FIG. 3A). Accordingly, for a multi-port structure (i.e., an N-port structure, where Nxe2x89xa72), method B can be quite time consuming. Further, in a fixtured or probing environment, a non-coaxial side of interface device 304 may be extremely space constrained so it may not be practical to construct the different line lengths required for method B. Since the differences in line length must be of the order xcex/4 at the operating frequency, this problem is particularly acute in the radio frequency (RF) ranges where the manufacturing need for such a method is greatest. Additionally, more line lengths than two are needed for broader frequency ranges.
A third technique, referred to hereafter as method C, uses two port calibrations at both ends of an interface device under test to determine the full S-parameters of the interface device. Method C is illustrated in FIG. 4 for a single interface device under test 404 (e.g., a fixture arm). While this process can work for a single interface device, it becomes quite time-consuming for N interface devices (e.g., an N-port fixture structure) and many calibrations maybe required. The following is a description of how method C would be used in a common two port problem.
Consider a wafer-probe environment where the desire is to find the S-parameters of the probes themselves (i.e., each probe is considered a two-port interface devices in this example). To implement method C, a pair of calibrations are required for each port, resulting in a total of four calibrations (although, all may not be needed in some cases). This basic example is shown in FIG. 5. Referring to FIG. 5, two interface devices 504a and 504b (i.e., wafer probe A and wafer probe B) are connected, respectively, to port 1 (P1) and port 2 (P2) of a VNA (not shown). As just mentioned, the desire is to find the S-parameters for the wafer probes (i.e., interface devices 504a and 504b) themselves.
The use of method C to find the S-parameters for the probes will now be described with reference to FIGS. 6A-6C. Using method C, a two port calibration at coaxial connector 506a is first performed. For this to make sense, the two wafer probes 504a and 504b must be connected by a thru 602, as shown in FIG. 6A. This is a critical step since this must be a true thru (i.e., no extra parasitics and reference planes in the wafer domain must be aligned). Further, extra length cannot be easily corrected for since there is an effect on the mismatch at the two wafer probes 504a and 504b. The next calibration is performed at the wafer level using commonly available calibration substrates 604, as shown in FIG. 6B. As just suggested, the reference planes established by this calibration must agree with the thru used in the first calibration. Also, the inflexibility associated with the interface device connection on the first calibration is important since there are often physical limitations on how this connection can be established with a real interface device (e.g., a real test fixture). The second calibration sequence needs a calibration at coaxial connector 506b, as shown in FIG. 6C, and the wafer level calibration can be re-used in this case to get the S-parameters of wafer probe 504b. 
To better understand the problem of wafer thru 602, consider the DUT outline shown in FIG. 7. In order to get the S-parameters of the DUT correctly, the reference planes for the second calibration should be established as shown (at the DUT terminals). A non-zero length thru is used during the second calibration (which is acceptable and correctable). However, when trying to create a true thru for the first and third calibrations, this presents a problem since there is a non-zero length between the desired reference planes. Accordingly, either some phase inaccuracy must be tolerated or the reference planes must be manually adjusted after the calibration, both of which are undesirable.
As discussed above, several methods exist for interface device characterization. Some are one port techniques that only reveal some of the S-parameters of the interface device. With some methods, all of the S-parameters are revealed, but several measurements and/or calibrations are typically required (especially for N-port fixtured or probed environments). Additionally, such methods may place constraints on the physical configurations of the interface devices. Accordingly, there is a need for a method for determining all relevant S-parameters of interface devices in a time-efficient and cost-efficient manner using a reduced number of measurements and/or calibrations.
The present invention is directed toward methods for determining the S-parameters of interface devices (e.g., adapters or test fixtures). The method of the present invention generates all S-parameters needed for de-embedding interface devices. Additionally, the present invention is geometrically feasible for a tight fixtured or probing environment. Further, the present invention is much more amenable for use in a multiport fixtured environment. Due to the increasing use of such multiport testing in manufacturing, a time efficient technique of extraction is quite important.
An embodiment of the present invention is directed to a method for characterizing N interface devices using a vector network analyzer (VNA). These N interface devices are useful for connecting an N-port device under test (DUT) to the VNA. A first step of the method includes performing an N-port calibration at each of N outer reference planes. Each outer reference plane is defined by a first port of one of the N interface devices. The first port of the each interface device is for connecting to the VNA (e.g., directly to a port of the VNA, or to a cable that is attached to the port of the VNA). The outer calibrations produce a set of outer reference plane error coefficients for each of the N interface devices, including a directivity error coefficient (edn), a source match error coefficient (epnS), and a reflection tracking error coefficient (etnn).
A second step of the method of the present invention includes performing an N-port calibration at each of N inner reference planes. Each of the inner reference planes is defined by a second port of one of the N interface devices. The second port of each interface is for connecting to one of the ports of the DUT. The inner calibrations produce a set of inner reference plane error coefficients for each of the N interface devices of the VNA, including a directivity error coefficient (ednxe2x80x2), a source match error coefficient (epnSxe2x80x2), and a reflection tracking error coefficient (etnnxe2x80x2).
After the inner and outer calibrations are performed, a set of S-parameters is determined for each of the N interface devices based on the results of the calibrations. More specifically, N sets of S-parameters are determined using equations that calculate S-parameters based on the error coefficients, such as edn, ednxe2x80x2, etc., determined during the calibrations. Each set of S-parameters characterizes a respective one of the N interface devices. The S-parameters of the interface devices can then be used to de-embed the interface devices from measured data, using conventional de-embedding techniques. If desired, the S-parameters can alternatively be used to embed interface devices into measured data.
Additional features and advantages of the present invention, as well as the operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.